Polycrystalline silicon thin film transistors are commonly used as basic electronic devices for controlling pixels of active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting displays (AMOLEDs). In addition, the polycrystalline silicon thin film transistors are also used as basic electronic devices required by the peripheral driving circuits and/or control circuits in these displays.
During production of the polycrystalline silicon thin film transistors (TFTs), the crystallization procedure for forming a polycrystalline silicon layer is critical. The electrical properties and uniformity of the polycrystalline silicon thin film transistors are primarily determined by this procedure.
A conventional process for forming a polycrystalline silicon layer using a crystallization method is illustrated with reference to FIGS. 1(a) and 1(b). Firstly, an amorphous silicon layer 104 is formed on a glass or plastic substrate 100. Then, the amorphous silicon layer 104 is irradiated with an excimer laser to be molten. The molten amorphous silicon layer is cooled and recrystallized into a polycrystalline silicon layer 105.
Referring to FIG. 2, the relationship between the energy density of an excimer laser irradiating on the amorphous silicon layer and the electrical property such as the electron mobility of the resulting recrystallized polycrystalline silicon layer is shown. As shown, the energy density effective for forming a polycrystalline silicon layer with good electrical property is confined to a narrow range. An excessive energy density may result in microcrystalline silicon rather than polycrystalline silicon. The electrical property of microcrystalline silicon is different from that of the polycrystalline silicon. For example, its electron mobility is not as high as required in some applications. On the other hand, an insufficient energy density substantially fails to effectively melt the amorphous silicon for subsequent recrystallization.
Furthermore, due to the variability of the excimer laser, the grain size of the recrystallized polycrystalline silicon layer 105 may be too small and non-uniform, and thus the electron property of the thin film transistor product varies significantly.